1. Field of the Invention
The present invention relates in general to a ceramic capacitor and more particularly concerns a ceramic capacitor which is suited to be used in high-voltage power instruments or equipment and is capable of withstanding a high impulse voltage such as that produced upon lightning stroke.
2. Description of the Prior Art
The ceramic capacitor generally comprises a column-like ceramic element formed by sintering a composition containing as a main component barium titanate, strontium titanate or the like and electrode layers of silver or the like fixedly deposited at end faces of the ceramic element. Further, an insulation material is coated over the outer peripheral surface of the ceramic element with a view to evading influences of various insulation media which will surround the ceramic capacitor placed for use. The diameter D as well as the thickness T of a ceramic element are determined depending on the desired electrostatic capacity. Usually, the dimensions D and T are not more than several ten millimeters because of a molding process effected under a high pressure in the manufacture of the ceramic element.
The severest characteristic requirement imposed on the ceramic capacitor destined to be used in a high voltage power instrument is a dielectric breakdown characteristic exhibited by the capacitor when a chopped-wave inpulse voltage produced upon lightning stroke is applied. In other words, the matter is such characteristic that the lowest dielectric breakdown field strength is attained at a chopping time (usually after a lapse of several micro-seconds) which field strength is considerably low compared with the dielectric breakdown field strength produced upon application of a full-wave impulse voltage.
As is known in the art, the voltage-time characteristic or V-t characteristic of most of conventional insulators is such that the dielectric breakdown field strength is increased as the time lapse from application of a full-wave voltage becomes less. However, in the case of the ceramic capacitor, the V-t characteristic is such that the dielectric breakdown field strength becomes lowered as the time lapse from application of the full-wave voltage becomes less. Such characteristic is unique to the ceramic capacitor and disadvantageous when the ceramic capacitor is to be used in power instruments.
Since selected test voltage values for conventional power instruments or equipment are generally higher for the chopped-wave voltage than for the full-wave voltage, the power instrument incorporating the ceramic capacitor is remarkably handicapped in regard to insulation.